In recent years, since problems of global warming due to carbon dioxide emission by use of fossil fuel and radioactive contamination by nuclear power plant accidents and radioactive waste have become serious, interests in global environment and energy are growing. Under these circumstances, solar power generation that uses solar light, i.e., an optical energy as an inexhaustible and clean energy source has been expected in the world.
Solar power generation apparatuses using solar batteries employ various forms corresponding to output scales from several W to several thousand kW. For general homes, solar power generation apparatuses that combine a 3- to 5-kW solar battery with a 3- to 5-kW inverter are generally used. In an apartment house or public facility where the solar battery installation area can be made larger than that for a general home, a solar power generation apparatus that combines a 10-kW solar battery with two to three 3- to 5-kW inverters connected in parallel is used.
A typical system using a solar battery converts (DC/AC-converts) DC power generated by a solar battery into AC power and supplies the AC power to a commercial power system. FIG. 8 is a circuit diagram showing the schematic arrangement of such a solar power generation apparatus.
Referring to FIG. 8, reference numeral 101 denotes a solar battery array constituted by connecting a plurality of solar battery modules in series to form solar battery strings and connecting the solar battery strings in parallel; 102, an inverter for executing DC/AC conversion; and 104, a commercial power system.
The DC output from the solar battery array 101 is collected by a current collecting box (not shown) and converted into commercial AC power by the inverter 102. A solar power generation apparatus is constructed by the solar battery array 101, current collecting box, and inverter 102. AC power generated by the solar power generation apparatus can be supplied to a load in home or to the commercial power system 104 through a distribution switchboard (not shown).
To prevent any electrical shock and to protect the current path in case of a DC ground fault in the solar battery array 101, a current transformer 105, control circuit 106, and system interconnection switch 108 are arranged in the conventional inverter 102.
A detection circuit in the current transformer 105 compares the value (ground fault detection value) of the differential current between the positive and negative poles of the solar battery array 101 with a predetermined threshold value, thereby detecting a ground fault in the solar battery array 101. If a ground fault is detected, the control circuit 106 stops an inverter circuit 107 in the inverter 102 and also turns off the system interconnection switch 108 to ensure the safety and protect the commercial power system.
In an arrangement disclosed in Japanese Patent Laid-Open No. 09-285015, an increase in capacitance between ground and a solar battery module due to water droplets sticking onto the solar battery module is taken into consideration. When a ground fault detection value representing a ground fault state is a predetermined value or more, the output power of the inverter circuit 107 is suppressed. If the ground fault detection value is still the predetermined value or more, the inverter circuit 107 is stopped, and the system interconnection switch 108 is turned off.
However, the conventional apparatus has the following disadvantages.
A voltage variation corresponding to the commercial AC frequency output from the inverter circuit 107 occurs in the DC current path of the inverter 102. This variation in voltage is transmitted to the DC current path of the solar battery array to cause a variation in AC current to ground (a variation in ground level). This variation in AC current to ground changes to an AC leakage current component by the capacitance of the solar battery array itself. The AC leakage current component increases as the current amount supplied to the inverter circuit, i.e., the power generation amount of the solar power generation apparatus increases.
In ground fault detection of the inverter, when a ground fault has occurred in the DC current path of the solar battery, the DC current path must be immediately disconnected from the commercial power system to prevent the ground fault current from continuously flowing. Hence, the sensitivity of ground fault detection of the inverter must be set high.
For this reason, in the conventional apparatus, even when the DC current path has no ground fault, an operation error may occur so that a ground fault state may be determined due to the influence of the AC leakage current component by an electrostatic capacitance and the inverter may be stopped.
Additionally, in the apparatus disclosed in Japanese Patent Laid-Open No. 09-285015, since the power generation amount is suppressed when the ground fault detection value increases due to the influence of the electrostatic capacitance, the total power generation amount of the solar power generation apparatus decreases.